I would love an extended cut, but I don't agree at all with what you said. Not to badmouth Heath Ledger's performance, or anything, but seriously, The Dark Knight is, imo, totally overrated, and the Joker right along with it. His motivation is pretty sketchy, and in no way is he fleshed out more than Harrison/Khan. That, I believe, is a matter of perspective.

Maybe the Joker wasn't more fleshed out, but he had the chance to show what he's capable of. He came across as a deadly threat, while nuKhan was nothing like that. He just wasn't menacing enough!